In nature, gilts (young female pigs that have not yet given birth to a litter of pigs) normally reach puberty between the ages of 150 and 250 days. Once they reach puberty, they have a regularly recurrent period of ovulation and sexual excitement, known as "estrus" or "heat." Gilts are most likely to ovulate and conceive when they are in heat.
Gilts and sows normally do not have synchronous heats (estrus), and accordingly, do not all conceive at the same time. Young gilts also have a tendency to produce smaller and lighter litters as compared to sows.
A major goal of commercial swine production is to maximize reproductive efficiency, especially among gilts. Increased reproductive efficiency offers producers substantial opportunities to reduce production costs and enhance profitability. Efforts are being made to increase reproductive efficiency by breeding gilts at earlier ages, synchronizing estrus among the gilts, impregnating gilts using artificial insemination (AI) and increasing the litter size and increasing the birth and weaning weight of the litters.
Gilts can be bred at earlier ages by chemically inducing puberty. One means to chemically induce puberty is by administration of a single injection of PG600.TM. (pregnant mare's serum gonadotrophin and human chorionic gonadotropin). Gilts typically show estrus between three and six days after treatment, while between 90 and 95% of the gilts ovulate even if they do not show estrus. The estrual response rate can be enhanced when gilts are given daily boar stimulation by direct physical contact beginning at the time of PG600.TM. administration.
The litter size of gilts bred at their first chemically induced estrus is comparable to the litter size in gilts bred at their first natural estrus. However, larger litter sizes can be obtained by waiting until the second or third estrus. A major problem associated with waiting for the second or third estrus is that the estrus is no longer closely synchronized.
There are no products currently approved in the United States that are effective at regulating estrus once gilts have started to cycle so that estrus synchronization during the more productive second or third estrus cycles is presently not possible. Estrus regulation and scheduled breedings are done using alternative management techniques, keeping large pools of gilts so that a fixed number will always be in heat or allowing nature to take its course. These techniques, however, are not efficient.
The most common alternative management technique is to synchronize estrus in groups of gilts by first mating the gilts and then administering Prostaglandin F2.alpha. (PGF2.alpha.) to induce "synchronous abortion" as soon as two to three weeks, and up to eight to ten weeks after the end of the mating period. Aborted gilts show a normal heat and normal fertility if abortion is induced during the first 40 to 50 days after mating. Gilts that have experienced an incomplete pregnancy tend to have larger litters than gilts bred at first or second estrus. This method of synchronizing estrus is presently used in some large swine operations, but it requires extra boars and extra boar housing. In addition, aborted conceptuses are unsanitary and may cause gilts to develop endometriosis after aborting.
There is still a heavy reliance on daily heat detection of individual animals for timing of AI or breeding, and gilts and sows are still bred based on spontaneous estrus cycles. Approximately half of the labor in swine breeding facilities is devoted to detection of heat in breeding gilts or sows. Gilts or sows must be checked at least once daily in order to breed at the correct time, and if AI is used it may be necessary to check heat twice daily to achieve the best results. Rigorous heat detection is necessary because it is difficult to predict the day of heat for any cyclic gilt or open sow, even with good heat detection records. Gilts often stay in the gilt pool for three or four cycles before they are first detected in heat or detected at the right time to fit a breeding group.
Methods to synchronize estrus and increase weaned litter weights in gilts would significantly improve the efficiency of swine production. Recent efforts have focused on the induction of estrus by first inducing pseudopregnancy in large numbers of animals and then inducing estrus in the animals. Pseudopregnancy is a condition resembling pregnancy that occurs in some mammals after infertile copulation. In a pseudopregnancy, physical symptoms of pregnancy, such as absence of sexual receptivity, weight gain and mammary development are manifested without conception.
Pseudopregnancy can be induced using existing or emerging commercial pharmaceutical products. Pseudopregnancy has been induced in cyclic gilts by giving estrogen for between four and eight days beginning on day 11 after heat (Geisert et al., "Length of pseudopregnancy and pattern of uterine protein release as influenced by time and duration of estrogen administration in the pig," J. Reprod. Fert. 79:163 (1987) and Pusateri, et al. "Maternal recognition of pregnancy in swine I: Minimal requirements for exogenous estradiol-17.beta. to induce short or long pseudopregnancy in cycling gilts," Biol. Reprod. 55: 582-589 (1996)). The compounds are typically administered by injection on a daily basis.
Pseudopregnant gilts maintain their corpus luteum (CL) for approximately 60 days and therefore can be induced to show heat "on demand" by treating with PGF2.alpha. during the pseudopregnancy (Geisert et al., (1987)). Heat usually occurs 3 to 6 days after PGF2.alpha. treatment in pseudopregnant gilts.
Daily injections or injected implants that have to be removed are impractical. It would be advantageous to provide a method for controlling the reproductive pathway for swine, especially gilts, that does not involve daily injections or the use of non biodegradable injected implants that need to be removed at the end of treatment.
It is therefore an object of the present invention to provide a method for controlling the reproductive pathway for swine, especially gilts, that does not involve daily injections or implants which have to be removed at the end of treatment.
It is another object of the present invention to provide a formulation for controlling the reproductive pathway of swine that uses a naturally-occurring estrogen or esters thereof that meets FDA approval.